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Arsenic species in weathering mine tailings and biogenic solids at the Lava Cap Mine Superfund Site, Nevada City, CA.

Foster AL, Ashley RP, Rytuba JJ - Geochem. Trans. (2011)

Bottom Line: Linear combination, least-squares fits constrained in part by PCA results were then used to quantify arsenic speciation in XAFS spectra of tailings and biogenic solids.The highest dissolved arsenic concentrations were found in Lost Lake porewater and in a groundwater-fed pond in the tailings deposition area.The stability of both primary and secondary As phases is likely to be at a minimum under cyclic wet-dry conditions.

View Article: PubMed Central - HTML - PubMed

Affiliation: U,S, Geological Survey, 345 Middlefield Rd,, MS 901 Menlo Park, CA, 94025, USA. afoster@usgs.gov.

ABSTRACT

Background: A realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived from historic mining operations is a major challenge to redevelopment of California's famed "Mother Lode" region. Arsenic, a known carcinogen, occurs in multiple solid forms that vary in bioaccessibility. X-ray absorption fine-structure spectroscopy (XAFS) was used to identify and quantify the forms of As in mine wastes and biogenic solids at the Lava Cap Mine Superfund (LCMS) site, a historic "Mother Lode" gold mine. Principal component analysis (PCA) was used to assess variance within water chemistry, solids chemistry, and XAFS spectral datasets. Linear combination, least-squares fits constrained in part by PCA results were then used to quantify arsenic speciation in XAFS spectra of tailings and biogenic solids.

Results: The highest dissolved arsenic concentrations were found in Lost Lake porewater and in a groundwater-fed pond in the tailings deposition area. Iron, dissolved oxygen, alkalinity, specific conductivity, and As were the major variables in the water chemistry PCA. Arsenic was, on average, 14 times more concentrated in biologically-produced iron (hydr)oxide than in mine tailings. Phosphorous, manganese, calcium, aluminum, and As were the major variables in the solids chemistry PCA. Linear combination fits to XAFS spectra indicate that arsenopyrite (FeAsS), the dominant form of As in ore material, remains abundant (average: 65%) in minimally-weathered ore samples and water-saturated tailings at the bottom of Lost Lake. However, tailings that underwent drying and wetting cycles contain an average of only 30% arsenopyrite. The predominant products of arsenopyrite weathering were identified by XAFS to be As-bearing Fe (hydr)oxide and arseniosiderite (Ca2Fe(AsO4)3O3•3H2O). Existence of the former species is not in question, but the presence of the latter species was not confirmed by additional measurements, so its identification is less certain. The linear combination, least-squares fits totals of several samples deviate by more than ± 20% from 100%, suggesting that additional phases may be present that were not identified or evaluated in this study.

Conclusions: Sub- to anoxic conditions minimize dissolution of arsenopyrite at the LCMS site, but may accelerate the dissolution of As-bearing secondary iron phases such as Fe3+-oxyhydroxides and arseniosiderite, if sufficient organic matter is present to spur anaerobic microbial activity. Oxidizing, dry conditions favor the stabilization of secondary phases, while promoting oxidative breakdown of the primary sulfides. The stability of both primary and secondary As phases is likely to be at a minimum under cyclic wet-dry conditions. Biogenic iron (hydr)oxide flocs can sequester significant amounts of arsenic; this property may be useful for treatment of perpetual sources of As such as mine adit water, but the fate of As associated with natural accumulations of floc material needs to be assessed.

No MeSH data available.


Related in: MedlinePlus

Representative k3-weighted EXAFS spectra of ore and tailings samples from group 1 (solid lines), corresponding least-squares fits (dotted line), and fit residual (offset line). Some spectra contain crystal glitches that were removed for PCA.
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Figure 5: Representative k3-weighted EXAFS spectra of ore and tailings samples from group 1 (solid lines), corresponding least-squares fits (dotted line), and fit residual (offset line). Some spectra contain crystal glitches that were removed for PCA.

Mentions: Initial fits were performed using the 4 model spectra with the lowest residuals in target transformation: for Group 1, these were arsenopyrite, As5+ sorbed to Fe3+ (hydr)oxide, aqueous As5+, and arseniosiderite. However, the contribution of the aqueous As5+ spectrum to Group 1 fits was consistently < 0.01% of the total. The resulting three-component fits were adequate for all but one spectrum: 97-LCD3, which is known from SEM evidence to be an ore specimen unusually rich in arsenian pyrite (see SEM micrograph in Additional File 1). Based on these results, the aqueous arsenate spectrum was removed from the set of model compounds used in linear combination, least-squares (LCLS) fits and replaced with that of arsenian pyrite, even though its target transform residual was higher than several other model compounds (e.g., scorodite and calcium arsenate). The Group 1 spectra were re-fit, and the resulting k3-weighted spectra, fits, and fit residuals are displayed in Figure 5 and Figure 6. The fits confirm that arsenian pyrite was the predominant As species in 97-LCD3 (69 ± 10%), but was at the limit of significance (1-6%) in fits to all other samples in Group 1 (Table 3; Figure 5).


Arsenic species in weathering mine tailings and biogenic solids at the Lava Cap Mine Superfund Site, Nevada City, CA.

Foster AL, Ashley RP, Rytuba JJ - Geochem. Trans. (2011)

Representative k3-weighted EXAFS spectra of ore and tailings samples from group 1 (solid lines), corresponding least-squares fits (dotted line), and fit residual (offset line). Some spectra contain crystal glitches that were removed for PCA.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3037876&req=5

Figure 5: Representative k3-weighted EXAFS spectra of ore and tailings samples from group 1 (solid lines), corresponding least-squares fits (dotted line), and fit residual (offset line). Some spectra contain crystal glitches that were removed for PCA.
Mentions: Initial fits were performed using the 4 model spectra with the lowest residuals in target transformation: for Group 1, these were arsenopyrite, As5+ sorbed to Fe3+ (hydr)oxide, aqueous As5+, and arseniosiderite. However, the contribution of the aqueous As5+ spectrum to Group 1 fits was consistently < 0.01% of the total. The resulting three-component fits were adequate for all but one spectrum: 97-LCD3, which is known from SEM evidence to be an ore specimen unusually rich in arsenian pyrite (see SEM micrograph in Additional File 1). Based on these results, the aqueous arsenate spectrum was removed from the set of model compounds used in linear combination, least-squares (LCLS) fits and replaced with that of arsenian pyrite, even though its target transform residual was higher than several other model compounds (e.g., scorodite and calcium arsenate). The Group 1 spectra were re-fit, and the resulting k3-weighted spectra, fits, and fit residuals are displayed in Figure 5 and Figure 6. The fits confirm that arsenian pyrite was the predominant As species in 97-LCD3 (69 ± 10%), but was at the limit of significance (1-6%) in fits to all other samples in Group 1 (Table 3; Figure 5).

Bottom Line: Linear combination, least-squares fits constrained in part by PCA results were then used to quantify arsenic speciation in XAFS spectra of tailings and biogenic solids.The highest dissolved arsenic concentrations were found in Lost Lake porewater and in a groundwater-fed pond in the tailings deposition area.The stability of both primary and secondary As phases is likely to be at a minimum under cyclic wet-dry conditions.

View Article: PubMed Central - HTML - PubMed

Affiliation: U,S, Geological Survey, 345 Middlefield Rd,, MS 901 Menlo Park, CA, 94025, USA. afoster@usgs.gov.

ABSTRACT

Background: A realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived from historic mining operations is a major challenge to redevelopment of California's famed "Mother Lode" region. Arsenic, a known carcinogen, occurs in multiple solid forms that vary in bioaccessibility. X-ray absorption fine-structure spectroscopy (XAFS) was used to identify and quantify the forms of As in mine wastes and biogenic solids at the Lava Cap Mine Superfund (LCMS) site, a historic "Mother Lode" gold mine. Principal component analysis (PCA) was used to assess variance within water chemistry, solids chemistry, and XAFS spectral datasets. Linear combination, least-squares fits constrained in part by PCA results were then used to quantify arsenic speciation in XAFS spectra of tailings and biogenic solids.

Results: The highest dissolved arsenic concentrations were found in Lost Lake porewater and in a groundwater-fed pond in the tailings deposition area. Iron, dissolved oxygen, alkalinity, specific conductivity, and As were the major variables in the water chemistry PCA. Arsenic was, on average, 14 times more concentrated in biologically-produced iron (hydr)oxide than in mine tailings. Phosphorous, manganese, calcium, aluminum, and As were the major variables in the solids chemistry PCA. Linear combination fits to XAFS spectra indicate that arsenopyrite (FeAsS), the dominant form of As in ore material, remains abundant (average: 65%) in minimally-weathered ore samples and water-saturated tailings at the bottom of Lost Lake. However, tailings that underwent drying and wetting cycles contain an average of only 30% arsenopyrite. The predominant products of arsenopyrite weathering were identified by XAFS to be As-bearing Fe (hydr)oxide and arseniosiderite (Ca2Fe(AsO4)3O3•3H2O). Existence of the former species is not in question, but the presence of the latter species was not confirmed by additional measurements, so its identification is less certain. The linear combination, least-squares fits totals of several samples deviate by more than ± 20% from 100%, suggesting that additional phases may be present that were not identified or evaluated in this study.

Conclusions: Sub- to anoxic conditions minimize dissolution of arsenopyrite at the LCMS site, but may accelerate the dissolution of As-bearing secondary iron phases such as Fe3+-oxyhydroxides and arseniosiderite, if sufficient organic matter is present to spur anaerobic microbial activity. Oxidizing, dry conditions favor the stabilization of secondary phases, while promoting oxidative breakdown of the primary sulfides. The stability of both primary and secondary As phases is likely to be at a minimum under cyclic wet-dry conditions. Biogenic iron (hydr)oxide flocs can sequester significant amounts of arsenic; this property may be useful for treatment of perpetual sources of As such as mine adit water, but the fate of As associated with natural accumulations of floc material needs to be assessed.

No MeSH data available.


Related in: MedlinePlus